问题
Let's say I have a string like this:
=====
and I want to replace it with this:
-----
I only want to replace it if it has more than a certain number of that character (we'll say > 3).
So, these should be the replacements:
=== -> ===
==== -> ----
===== -> -----
The application is I want to replace all level 1 heading marks in markdown with a level 2 mark, without changing embedded code blocks.
I know I can do this:
/=/-/g, but this matches anything with an equals sign (if (x == y)), which is undesirable.
or this:
/===+/----/g, but this doesn't account for the length of the original matched string.
Is this possible?
回答1:
It's possible with Perl:
my $string = "===== Hello World ====";
$string =~ s/(====+)/"-" x length($1)/eg;
# $string contains ----- Hello World ----
Flag /e makes Perl execute expression in second part of s///. You may try this with oneliner:
perl -e '$ARGV[0] =~ s/(====+)/"-" x length($1)/eg; print $ARGV[0]' "===== Hello World ===="
回答2:
Depending what language you're using. Basically, in some languages, you can put code in the right side of the regexp, allowing you to do something like this: (this is in perl):
s/(=+)/(length($1) > 3 ? "-" : "=") x length($1)/e
The 'e' flag tells perl to execute the code in the right side of the expression instead of just parsing it as a string.
来源：https://stackoverflow.com/questions/7337995/regex-replace-sequence-of-one-character-with-same-number-of-another-character